1) Field of the Invention
The present invention relates to an optical fiber and an optical transmission line using the optical fiber. More particularly, the present invention relates to an optical fiber and an optical transmission line for use in wavelength division multiplexing (WDM) optical transmissions.
2) Description of the Related Art
Technologies for increasing transmission capacities of optical transmissions using optical fibers have been actively studied in the art.
In general, a transmission loss in the optical fiber is minimized at a wavelength of approximately 1550 nanometers. Therefore, in the optical transmissions using optical fibers, the use of this wavelength band is desired, and a dispersion-shifted fiber (DSF) having a zero dispersion wavelength at a wavelength of approximately 1550 nanometers has been developed. This optical fiber allows an optical transmission with a transmission capacity of several gigabits/sec (Gbps) to be achieved in a wavelength band of 1550 nanometers.
In recent years, as the technologies for increasing the transmission capacities, wavelength division multiplexing (WDM) optical transmissions have been extensively researched and developed. In addition, optical fibers suitable for WDM optical transmissions have been widely studied.
From the viewpoint of preventing the four-wave mixing that is one of non-linear phenomena, optical fibers for WDM optical transmissions are required to have no zero dispersion wavelength in a transmission wavelength band. This requirement results in development of a Non-Zero Dispersion-Shifted Fiber (hereinafter, “NZDSF”). The development of the NZDSF allows WDM transmissions in a wavelength range of 1530 to 1565 nanometers (C band) and a wavelength range of 1565 to 1625 nanometers (L band) to be achieved with remarkably increased transmission capacities.
In such WDM transmission systems, widening of the wavelength band of signal light has been attempted to further expand transmission capacities to a shorter wavelength range of 1460 to 1530 nanometers (S band).
Recently, to extend the transmission band of the wavelength division multiplexing transmission, it is studied to achieve a broadband optical amplifier using an Er-doped optical fiber amplifier (EDFA). In addition, applications of Raman amplifiers and new optical amplifiers using optical fibers doped with a rare earth element have been actively researched for the wavelength division multiplexing transmission, and even discussion for practical use has been started. Among those, a Raman-amplified optical transmission line using the Raman amplifier is expected to be actually available soon.
Raman amplification is described briefly below. In general, an incident light to a substance such as glass generates a light with a slightly longer wavelength than an original wavelength due to molecular oscillations and lattice oscillations of the substance. This generated light is called Raman scattered light. A more intensive incident pump light can generate more intensive Raman scattered lights with identical phases. The lights are called induced Raman scattered lights.
If an incident signal light has the same wavelength as that of the induced Raman scattered light, the induced Raman scattered light undergoes the same intensity variation as that of the signal light and thus amplifies the signal light. Due to induced Raman scattering that is caused when an intensive light (pump light) enters an optical fiber, a gain appears at a longer wavelength by approximately 100 nanometers than the pump light wavelength. This phenomenon is utilized in Raman amplification. That is, the pump light that has the gain in the wavelength range of the signal light is incident to the optical fiber for amplifying the signal light. Raman amplification is such a method of amplifying optical signals. A fiber-optic amplifier using Raman amplification utilizes an amplification due to a non-linear phenomenon that is caused in such the optical fiber.
As Raman amplification is caused in an optical transmission line (optical fiber), it is also referred to as a system that provides the optical transmission line with an amplifying light other than a signal light.
The conventional optical fibers include, for the purpose of achieving a broadband, an optical fiber having a zero dispersion wavelength shifted to around 1380 nanometers that is an absorption band for a hydroxyl group (OH group), and having an effective area, Aeff, of not less than 60 μm2 at a wavelength of 1550 nanometers to prevent occurrence of four-wave mixing in dense wavelength division multiplexing (DWDM) (see U.S. Pat. No. 6,266,467, for example).
As known generally, however, an increased effective area Aeff lowers the efficiency for Raman amplification.
Aeff is defined in the following equation (1):
                    Aeff        =                                            (                                                ∫                  0                  ∞                                ⁢                                                      E                    2                                    ·                  r                  ·                                                                          ⁢                                      ⅆ                    r                                                              )                        2                                              ∫              0              ∞                        ⁢                                          E                4                            ·              r              ·                                                          ⁢                              ⅆ                r                                                                        (        1        )            where E denotes an electric field associated with a propagating light, and r denotes a distance in the radial direction from the core center in an optical fiber.
Raman amplification for the optical fiber disclosed in the patent requires a large optical power equal to or more than one Watt in total as a pump source.
This requirement not only simply lowers the efficiency uneconomically but also adversely affects an optical connector connected to the transmission line and a resin coated over the optical fiber possibly due to heat caused from application of a high power.
As shown, the optical fiber disclosed in the patent is not suitable for Raman-amplified transmission systems.
The patent describes that the broadband transmission in a wavelength range of 1200 to 1600 nanometers can be achieved by reducing the transmission loss in the OH group absorption peak band a minimum.
In general, the OH group absorption peak is known to increase with time when hydrogen enters. The patent, however, fails to give a description on the stable use of optical fibers for a long term.
The optical fiber, to the contrary, is known to have a reduced effective area Aeff for light propagating if the signal optical transmission band is shifted to a shorter wavelength. Accordingly, on transmission of a signal light in S band (1460 to 1530 nanometers) in the WDM system, an increase in the number of signals propagating in an optical fiber elevates the light intensity per unit area, resulting in a problem associated with signal distortion caused from the non-linear phenomenon.
Another attempt is made to expand the signal optical transmission band to a longer wavelength of 1625 to 1700 nanometers including U band (1625 to 1675 nanometers).